Fix handling of archive files with upper-case extensions (e.g. .DOCX)

[pmyteh]

In identifying the correct filetype to read, readtext() uses tolower() on the file extension. But extract_archive() doesn't. So if you try to read a file with the extension ".DOCX" it is correctly sent to extract_archive(), but that fails with "Archive extension DOCX unrecognised." and the file cannot be read. This trivial PR fixes that.

[kbenoit]

Thanks @pmyteh!